The beaches of Delaware Bay are prime territory for the spawning of Horseshoe Crabs, an ancient species of Arthropod more closely related to spiders than to other types of crabs or crustaceans. The best time to observe Horseshoe Crab spawning is in the evening, at high tide, near the full and new moons of May and early June.
Playlist of videos about the bay (just horseshoe crabs for now).